Methods of fabricating solid oxide fuel cells

ABSTRACT

In various embodiments, a solid oxide fuel cell is fabricated in part by disposing a functional layer between the cathode and the solid electrolyte.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/310,358, filed Mar. 18, 2016, the entiredisclosure of which is hereby incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with United States Government support underContract No. DEAR0000494 awarded by the Department of Energy AdvancedProjects Research Agency-Energy (ARPA-E). The United States Governmenthas certain rights in the invention.

TECHNICAL FIELD

In various embodiments, the present invention relates to methods offabricating solid oxide fuel cells, and in particular to solid oxidefuel cells with cathode-side functional interlayers.

BACKGROUND

Solid oxide fuel cells (SOFCs) are highly efficient, environmentallyfriendly electrochemical devices that are capable of directly convertingchemical energy stored in hydrogen or hydrocarbon fuels into electricalenergy. During SOFC operation, oxygen ions migrate from a cathode to ananode through a dense electrolyte. At the anode, the oxygen ions oxidizethe fuel, resulting in the generation of electrons that may be directedthrough an external circuit. SOFCs may be used for, for example,off-grid and portable power generation.

Although SOFCs have many advantages over other types of fuel cells, manyconventional SOFCs operate at high temperatures (e.g., 800° C. orabove), which can lead to degradation of the cell components, longerstartup times, and high maintenance costs. While lower-temperatureoperation would address many of these issues, operation at lowertemperatures also presents operational challenges. For example, internallosses within the electrolyte and cathode resistance are typicallyhigher during operation at lower temperature. Thus, there is a need forSOFC structures that have improved performance during low-temperatureoperation.

SUMMARY

In accordance with various embodiments of the present invention, SOFCsincorporate thin functional layers between the electrolyte and thecathode in order to reduce the interfacial resistance between theelectrolyte and the cathode. While not wishing to be bound by anyparticular theory of operation, the present inventors believe thatfunctional layers in accordance with embodiments of the presentinvention enhance the bonding and/or adhesion at the interface betweenthe cathode and the solid electrolyte, thereby expediting oxygen iontransport within the SOFC. Exemplary functional layers in accordancewith embodiments of the invention include cobalt-doped gadolinium-dopedceria (Co-GDC, for example, Co—Ce_(0.9)Gd_(0.1)O_(1.95)) andcobalt-doped samarium doped ceria (Co-SDC, for example,Co—Ce_(0.8)Sm_(0.2)O_(1.9)). The functional layer may be disposed and indirect contact with the solid electrolyte and the cathode material, andthe functional layer may be fabricating utilizing, for example, wetchemical or solid-state techniques.

Functional layers in accordance with embodiments of the presentinvention may also provide one or more additional advantages for SOFCoperation. For example, various functional layers (e.g., Co-GDC) havehigh ionic transference numbers (i.e., fractions of their total currentcarrying capability carried via ionic transport), and thus may beutilized to retard, block, or substantially minimize electronicconduction in the SOFC when utilized with mixed-conducting electrolytes(i.e., electrolytes that are both ionic and electronic conductors) suchas GDC. This reduction in electronic conduction may advantageouslyimprove, for example, the open-circuit voltage of the SOFC.

Furthermore, functional layers in accordance with embodiments of theinvention may have high oxygen exchange coefficients, thereby enablingfaster surface oxygen adsorption and desorption, and thus faster oxygenlattice incorporation. Thus, the functional layer may enhance masstransport at the interface between the cathode and the electrolyte,reducing interfacial resistance to oxygen introduction into theelectrolyte, increasing SOFC performance, and decreasing thearea-specific resistance of the SOFC. Moreover, the increased oxygenexchange coefficients of functional layers in accordance withembodiments of the invention advantageously enable lower-temperatureSOFC operation. For example, oxygen surface exchange within variousfunctional layers may occur at temperatures as low as approximately 400°C., enabling SOFC operation at temperatures ranging from, for exampleapproximately 400° C. to approximately 750° C., or even approximately400° C. to approximately 550° C. In accordance with various embodimentsof the invention, high oxygen surface coefficients are enabled by theagglomeration or redistribution of one or more elements of thefunctional layer (e.g., a dopant such as Co) toward or at grainboundaries in the functional layer, thereby forming dopant-rich phaseswithin the functional layer.

In addition, constituent elements and/or species within variousfunctional layers in accordance with embodiments of the invention act assintering agents that enable increased densification of the functionallayer at lower temperatures. For example, Co-GDC may be densified attemperatures as low as 900° C., while GDC requires temperatures ofapproximately 1400° C. for densification. The ability to densifyfunctional layers at lower temperatures maintains a greater variety ofmaterials compatibility options with regard to other materials withinthe SOFC (e.g., with cathode and electrode materials for chemicalcompatibility to avoid materials reactions between layers) and may alsohelp to minimize thermal-mismatch issues resulting from differentcoefficients of thermal expansion between layers (i.e., stresses thanmay arise during temperature cycling during, e.g., fabrication and/oroperation, due to differential contraction and/or expansion). The use ofhigh-density functional layers may also minimize or reduce ohmic lossesthat might arise due to the use of porous or poorly sintered layers.Finally, the ability to sinter functional layers in accordance withembodiments of the invention at lower temperatures may also prevent orreduce diffusion of various elements from the functional layer (e.g., Coor other metals) into the electrolyte of the SOFC, thereby maintaininghigher performance of the SOFC. In fact, various embodiments of theinvention feature SOFCs with multilayer electrolytes, in which theconcentration of a mobile species (e.g., a metal such as Co) within afirst layer of the electrolyte (where the first electrolyte is closer tothe cathode) is higher than that within a second layer of theelectrolyte (where the second electrolyte is closer to the anode), wherethe first layer of the electrolyte is adjacent to the functional layer.In such SOFCs, the concentration of the mobile species is higher still(i.e., higher than in any layer of the electrolyte) within thefunctional layer. Such structures may advantageously resistinterdiffusion of the mobile species and thereby maintain high SOFCperformance.

In an aspect, embodiments of the invention feature a solid oxide fuelcell that includes, consists essentially of, or consists of a cathode, asolid electrolyte, an anode, and a functional layer. The solidelectrolyte conducts oxygen ions from the cathode to the anode. At theanode, the oxygen ions react with a hydrogen-containing fuel to generateelectricity. The functional layer is disposed between the cathode andthe solid electrolyte. The functional layer reduces interfacialresistance (to, e.g., ionic conduction) between the cathode and thesolid electrolyte.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The functional layer may be disposedin direct contact with the cathode and/or with the solid electrolyte.The functional layer may include, consist essentially of, or consist ofcobalt-doped gadolinium-doped ceria and/or cobalt-doped samarium-dopedceria. The cathode may include, consist essentially of, or consist oflanthanum strontium cobalt ferrite, lanthanum strontium manganite,lanthanum strontium cobaltite, barium strontium cobalt ferrite, samariumstrontium cobaltite, samarium-doped ceria, and/or gadolinium-dopedceria. The solid electrolyte may include, consist essentially of, orconsist of yttria-stabilized zirconia, scandia-stabilized zirconia,lanthanum strontium magnesium gallate, samarium-doped ceria, and/orgadolinium-doped ceria. The anode may include, consist essentially of,or consist of a composite that includes, consists essentially of, orconsists of one or more metals (e.g., nickel) and/or one or more metaloxides (e.g., nickel oxide) and a material of the solid electrolyte(e.g., gadolinium-doped ceria). The anode may include, consistessentially of, or consist of a composite that includes, consistsessentially of, or consists of one or more metals (e.g., nickel) and/orone or more metal oxides (e.g., nickel oxide) and yttria-stabilizedzirconia. The thickness of the functional layer may range fromapproximately 0.1 μm to approximately 20 μm, from approximately 0.5 μmto approximately 15 μm, from approximately 1 μm to approximately 12 μm,from approximately 1 μm to approximately 10 μm, or from approximately 1μm to approximately 6 μm. The hydrogen-containing fuel may include,consist essentially of, or consist of hydrogen and/or a hydrocarbon. Thefunctional layer may contain one or more metals or other dopants (e.g.,cobalt, samarium, etc.) at a composition ranging from approximately 0.1mol % to approximately 10 mol %, from approximately 0.2 mol % toapproximately 8 mol %, or from approximately 0.5 mol % to approximately5 mol %. The functional layer and at least a portion of the solidelectrolyte (e.g., one or more layers of an electrolyte composed ofmultiple layers, or a portion of a single-layer electrolyte) may bothcontain one or more metals or other dopants (e.g., cobalt, samarium,etc.). The concentration of at least one said metal or other dopant(e.g., cobalt, samarium, etc.) within the at least a portion of thesolid electrolyte may be less than the concentration within thefunctional layer. One or more layers of the solid electrolyte may have aconcentration of at least one said metal or other dopant (e.g., cobalt,samarium, etc.) greater than that within one or more other portions orlayers of the solid electrolyte and/or less than that within thefunctional layer.

In another aspect, embodiments of the invention feature a method ofelectrochemically converting a hydrogen-containing fuel to electricityusing a solid oxide fuel cell that includes, consists essentially of, orconsists of (i) a cathode, (ii) an anode, (iii) a solid electrolytedisposed between the anode and the cathode, and (iv) a functional layerdisposed between the cathode and the solid electrolyte. Oxygen or anoxygen-containing gas is ionized at the cathode, thereby producingoxygen ions. The oxygen ions are conducted from the cathode to theanode, through the solid electrolyte, whereby the functional layerreduces interfacial resistance between the cathode and the solidelectrolyte. The oxygen ions are reacted with the fuel at the anode,thereby generating electricity.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The functional layer may be disposedin direct contact with the cathode and/or with the solid electrolyte.The functional layer may include, consist essentially of, or consist ofcobalt-doped gadolinium-doped ceria and/or cobalt-doped samarium-dopedceria. The cathode may include, consist essentially of, or consist oflanthanum strontium cobalt ferrite, lanthanum strontium manganite,lanthanum strontium cobaltite, barium strontium cobalt ferrite, samariumstrontium cobaltite, samarium-doped ceria, and/or gadolinium-dopedceria. The solid electrolyte may include, consist essentially of, orconsist of yttria-stabilized zirconia, scandia-stabilized zirconia,lanthanum strontium magnesium gallate, samarium-doped ceria, and/orgadolinium-doped ceria. The anode may include, consist essentially of,or consist of a composite that includes, consists essentially of, orconsists of one or more metals (e.g., nickel) and/or one or more metaloxides (e.g., nickel oxide) and a material of the solid electrolyte(e.g., gadolinium-doped ceria). The anode may include, consistessentially of, or consist of a composite that includes, consistsessentially of, or consists of one or more metals (e.g., nickel) and/orone or more metal oxides (e.g., nickel oxide) and yttria-stabilizedzirconia. The thickness of the functional layer may range fromapproximately 0.1 μm to approximately 20 μm, from approximately 0.5 μmto approximately 15 μm, from approximately 1 μm to approximately 12 μm,from approximately 1 μm to approximately 10 μm, or from approximately 1μm to approximately 6 μm. The hydrogen-containing fuel may include,consist essentially of, or consist of hydrogen and/or a hydrocarbon. Thefunctional layer may contain one or more metals or other dopants (e.g.,cobalt, samarium, etc.) at a composition ranging from approximately 0.1mol % to approximately 10 mol %, from approximately 0.2 mol % toapproximately 8 mol %, or from approximately 0.5 mol % to approximately5 mol %. The functional layer and at least a portion of the solidelectrolyte (e.g., one or more layers of an electrolyte composed ofmultiple layers, or a portion of a single-layer electrolyte) may bothcontain one or more metals or other dopants (e.g., cobalt, samarium,etc.). The concentration of at least one said metal or other dopant(e.g., cobalt, samarium, etc.) within the at least a portion of thesolid electrolyte may be less than the concentration within thefunctional layer. One or more layers of the solid electrolyte may have aconcentration of at least one said metal or other dopant (e.g., cobalt,samarium, etc.) greater than that within one or more other portions orlayers of the solid electrolyte and/or less than that within thefunctional layer. The oxygen ions may be reacted with the fuel at atemperature ranging from approximately 300° C. to approximately 800° C.,from approximately 350° C. to approximately 750° C., from approximately400° C. to approximately 750° C., from approximately 400° C. toapproximately 700° C., from approximately 450° C. to approximately 650°C., from approximately 450° C. to approximately 600° C., or fromapproximately 500° C. to approximately 650° C.

In yet another aspect, embodiments of the invention feature a method offabricating a solid oxide fuel cell. An anode layer is provided. A solidelectrolyte layer is disposed over the anode layer. A functional layeris disposed (e.g., deposited) over the solid electrolyte layer. Acathode layer is disposed over the functional layer, thereby forming thesolid oxide fuel cell.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The functional layer may be disposedin direct contact with the cathode layer and/or with the solidelectrolyte layer. The functional layer may include, consist essentiallyof, or consist of cobalt-doped gadolinium-doped ceria and/orcobalt-doped samarium-doped ceria. The cathode layer may include,consist essentially of, or consist of lanthanum strontium cobaltferrite, lanthanum strontium manganite, lanthanum strontium cobaltite,barium strontium cobalt ferrite, samarium strontium cobaltite,samarium-doped ceria, and/or gadolinium-doped ceria. The solidelectrolyte layer may include, consist essentially of, or consist ofyttria-stabilized zirconia, scandia-stabilized zirconia, lanthanumstrontium magnesium gallate, samarium-doped ceria, and/orgadolinium-doped ceria. The anode layer may include, consist essentiallyof, or consist of a composite that includes, consists essentially of, orconsists of one or more metals (e.g., nickel) and/or one or more metaloxides (e.g., nickel oxide) and a material of the solid electrolyte(e.g., gadolinium-doped ceria). The anode layer may include, consistessentially of, or consist of a composite that includes, consistsessentially of, or consists of one or more metals (e.g., nickel) and/orone or more metal oxides (e.g., nickel oxide) and yttria-stabilizedzirconia. The thickness of the functional layer may range fromapproximately 0.1 μm to approximately 20 μm, from approximately 0.5 μmto approximately 15 μm, from approximately 1 μm to approximately 12 μm,from approximately 1 μm to approximately 10 μm, or from approximately 1μm to approximately 6 μm. The hydrogen-containing fuel may include,consist essentially of, or consist of hydrogen and/or a hydrocarbon. Thefunctional layer may contain one or more metals or other dopants (e.g.,cobalt, samarium, etc.) at a composition ranging from approximately 0.1mol % to approximately 10 mol %, from approximately 0.2 mol % toapproximately 8 mol %, or from approximately 0.5 mol % to approximately5 mol %. The functional layer and at least a portion of the solidelectrolyte layer (e.g., one or more layers of an electrolyte layercomposed of multiple layers, or a portion of a single-layer electrolytelayer) may both contain one or more metals or other dopants (e.g.,cobalt, samarium, etc.). The concentration of at least one said metal orother dopant (e.g., cobalt, samarium, etc.) within the at least aportion of the solid electrolyte layer may be less than theconcentration within the functional layer. One or more layers of thesolid electrolyte layer may have a concentration of at least one saidmetal or other dopant (e.g., cobalt, samarium, etc.) greater than thatwithin one or more other portions or layers of the solid electrolytelayer and/or less than that within the functional layer.

The solid electrolyte layer may be disposed over the anode layer beforethe functional layer is deposited. The anode layer and/or the solidelectrolyte layer may be provided by tape casting. Disposing the solidelectrolyte layer over the anode layer may include, consist essentiallyof, or consist of laminating the solid electrolyte layer to the anodelayer (by, e.g., hot pressing and/or cold pressing). The solidelectrolyte may be provided or formed before the solid electrolyte layeris disposed over the anode layer. The solid oxide fuel cell may beannealed after the cathode layer is disposed over the functional layer.At least a portion of the solid oxide fuel cell (e.g., the anode layerand the solid electrolyte layer; the anode layer, the solid electrolytelayer, and the functional layer; or the anode layer, the solidelectrolyte layer, the functional layer, and the cathode layer) may beannealed after the solid electrolyte layer is disposed over the anodelayer. The functional layer and the cathode layer may be annealedtogether (e.g., co-fired) after the cathode layer is disposed over thefunctional layer. The solid oxide fuel cell may be annealed at atemperature ranging from approximately 600° C. to approximately 1300°C., from approximately 650° C. to approximately 1250° C., fromapproximately 700° C. to approximately 1200° C., from approximately 750°C. to approximately 1150° C., from approximately 800° C. toapproximately 1100° C., from approximately 850° C. to approximately1050° C., from approximately 900° C. to approximately 1000° C., or fromapproximately 900° C. to approximately 950° C. The solid oxide fuel cellmay be annealed at a temperature no greater than approximately 1100° C.,no greater than approximately 1050° C., no greater than approximately1000° C., or no greater than approximately 950° C. The solid oxide fuelcell may be annealed at a temperature no less than approximately 800°C., no less than approximately 850° C., or no less than approximately900° C. The solid oxide fuel cell may be annealed for a time periodranging from approximately 0.3 hour to approximately 10 hours, fromapproximately 0.4 hour to approximately 8 hours, from approximately 0.5hour to approximately 5 hours, or from approximately 1 hour toapproximately 4 hours. The solid oxide fuel cell may be annealed for atime period no greater than 20 hours. The solid oxide fuel cell may beannealed for a time period no greater than 10 hours. The solid oxidefuel cell may be annealed for a time period no less than 0.5 hours. Thesolid oxide fuel cell may be annealed for a time period no less than 1hour. The thickness of the functional layer may be selected based atleast in part on an intended temperature of operation (e.g., thetemperature of the fuel and/or of the solid oxide fuel cell itself) ofthe solid oxide fuel cell.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately” and “substantially” mean ±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. For example, a structure consistingessentially of multiple metals will generally include only those metalsand only unintentional impurities (which may be metallic ornon-metallic) that may be detectable via chemical analysis but do notcontribute to function.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic diagram of solid oxide fuel cell incorporating afunctional layer in accordance with various embodiments of theinvention;

FIGS. 2A and 2B are schematic diagrams of test structures for theevaluation of cathode performance in accordance with various embodimentsof the invention;

FIG. 3 is a graph of area specific resistance for various cathode teststructures in accordance with various embodiments of the invention;

FIG. 4 is a graph of cell performance at various operating temperaturesfor a cell incorporating a 6 μm thick functional layer in accordancewith various embodiments of the invention;

FIG. 5 is a graph of cell performance at various operating temperaturesfor a cell incorporating a 1 μm thick functional layer in accordancewith various embodiments of the invention;

FIG. 6 is a graph of cell performance at an operating temperature of650° C. for fuel cells with functional layers of various thicknesses inaccordance with various embodiments of the invention;

FIG. 7 is a graph of cell performance at an operating temperature of500° C. for fuel cells with functional layers of various thicknesses inaccordance with various embodiments of the invention; and

FIG. 8 is a graph of oxygen surface exchange coefficients of twoexemplary materials in accordance with various embodiments of theinvention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a SOFC 100 in accordance with embodimentsof the present invention. As shown, the SOFC 100 features a cathode 110,a solid electrolyte 120, an anode 130, and a functional layer 140.During operation of SOFC 100, oxygen from an oxygen source 150 (e.g.,air) is ionized by the cathode 110. The resulting oxygen ions areconducted from the cathode 110, through the solid electrolyte 120, tothe anode 130. At the anode 130, the oxygen ions are reacted with ahydrogen-containing fuel 160 to produce electricity. As shown, thegenerated electricity may be flowed through an external load 170 andback to the cathode 110 to support further ionization at the cathode110. The electrochemical reaction may also generate by-products such as,e.g., water and carbon dioxide at the anode 130. The hydrogen-containingfuel 160 may include, consist essentially of, or consist of, forexample, hydrogen gas and/or a hydrocarbon fuel such as natural gas,propane, gasoline, diesel, or biofuel. In various embodiments, thehydrogen-containing fuel 160 may include, consist essentially of, orconsist of a mixture of H₂, CO, H₂O, CO₂, and CH₄. For increased powergeneration, multiple SOFCs 100 may be linked together in a stackedstructure. While FIG. 1 depicts SOFC 100 in a planar configuration, invarious embodiments of the invention the SOFC may be arranged in aconcentric tubular configuration. For example, the oxygen source 150 maybe flowed through a tubular cathode 110, around which are disposedtubular electrolyte 120 and anode 130 (with a tubular functional layer140 disposed between the cathode 110 and the solid electrolyte 120),while the hydrogen-containing fuel 160 may be flowed around the outsideof the anode 130.

As detailed herein, the presence and use of the functional layer 140within SOFC 100 may provide various beneficial effects enhancing theoverall performance of SOFC 100. For example, the functional layer 140may reduce interfacial resistance between the cathode 110 and theelectrolyte 120. In various embodiments, the functional layer 140 has anoxygen exchange coefficient higher than that of the electrolyte 120and/or the cathode 110, and therefore enhances oxygen transport withinSOFC 100. As a result of faster oxygen transport and/or lowerinterfacial resistance, the functional layer 140 may also decrease thearea-specific resistance of the SOFC 100. Functional layers 140 may alsoretard or block electronic conduction, as they may have ionictransference numbers higher than those for the electrolyte 120. Inaccordance with various embodiments of the invention, the functionallayer 140 may have a thickness ranging from approximately 0.1 μm toapproximately 20 μm, from approximately 1 μm to approximately 15 μm, orfrom approximately 1 μm to approximately 10 μm. In various embodiments,the thickness of the functional layer 140 is no greater than 20 μm, asthicker functional layers may exhibit deleteriously high ohmicresistance and thus limit the performance of SOFC 100. In variousembodiments, the thickness of the functional layer 140 is no less than0.1 μm, as thinner functional layers may not be fully continuous and maytherefore not provide beneficial effects.

In accordance with embodiments of the invention, the presence of thefunctional layer 140 permits efficient operation of the SOFC 100 atlower-than-conventional temperatures. For example, SOFC 100 may beoperated at temperatures below approximately 800° C., e.g., attemperatures ranging from approximately 400° C. to approximately 750° C.Moreover, the thickness of the functional layer 140 may be selectedbased on, at least in part, the desired operating temperature for theSOFC 100. For example, at relatively higher temperatures (e.g., rangingfrom approximately 550° C. to approximately 750° C.), the performance ofthe SOFC 100 may be limited by the open-circuit voltage, and thusrelatively thicker functional layers 140 (for example, 5 μμm or thicker,e.g., a thickness ranging from approximately 5 μm to approximately 20μm, or ranging from approximately 5 μm to approximately 10 μm) may beutilized to enhance the open-circuit voltage and thus the overallperformance of the SOFC 100. Alternatively, at relatively lowertemperatures (e.g., ranging from approximately 400° C. to approximately550° C.), the performance of the SOFC 100 may be limited by ohmicresistance (that is, although the presence of the functional layer 140still reduces the interfacial resistance between the cathode 110 and theelectrolyte 120, the ohmic resistance of the functional layer 140 itselfmay impact performance at lower temperatures), and thus relativelythinner functional layers 140 (for example, 5 μm or thinner, e.g., athickness ranging from approximately 0.1 μm to approximately 5 μm, orranging from approximately 1 μm to approximately 5 μm) may be utilizedto minimize or reduce ohmic resistance and thus improve the overallperformance of the SOFC 100.

In various embodiments of the invention, the cathode 110 includes,consists essentially of, or consists of, for example, one or more oflanthanum strontium cobalt ferrite (LSCF) (e.g.,La_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O_(3-δ)), lanthanum strontium manganite(LSM) (e.g., La_(0.8)Sr_(0.2)MnO_(3-δ)), lanthanum strontium cobaltite(LSC) (e.g., La_(0.6)Sr_(0.4)CO_(3-δ)), barium strontium cobalt ferrite(BCSF) (e.g., Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ)), samariumstrontium cobaltite (SSC) (e.g., Sm_(0.5)Sr_(0.5)CoO_(3-δ)), or dopedceria with dopant concentrations ranging from 5 to 30 mol % (or rangingfrom 10 to 20 mol %), for example samarium-doped ceria (SDC) (e.g.,Co-Ce_(0.8)Sm_(0.2)O_(1.9)), gadolinium-doped ceria (GDC) (e.g.,Co—Ce_(0.9)Gd_(0.1)O_(1.95)), or ceria doped with any one or more of Y,La, Pr, Sm, or Nd.

For example, the cathode 110 may include, consist essentially of, orconsist of a mixture of SSC and GDC, e.g., in a ratio of 3:7 to 7:3 bymass. The cathode 110 may be both an ionic conductor and an electronicconductor, and the cathode 110 may be porous to promote oxygen accessfor ionization and to provide electrochemically active triple phaseboundaries (TPBs) where the electrolyte 120 (or functional layer 140),air, and cathode 110 meet. For example, the cathode 110 may have aporosity ranging from approximately 30% to approximately 60%.

In general, the solid electrolyte 120 is a dense (e.g., ceramic)material that conducts oxygen ions while minimizing electronicconduction therewithin in order to prevent current leakage andcorresponding electrical losses. The thickness of the solid electrolyte120 may range from, for example, approximately 5 μm to approximately 40μm, or even from approximately 10 μm to approximately 30 μm. In variousembodiments of the invention, the solid electrolyte 120 includes,consists essentially of, or consists of, for example, one or more ofyttria-stabilized zirconia (YSZ, e.g., Y_(0.16)Zr_(0.84)O_(2-x)),scandia-stabilized zirconia (ScSZ, e.g.,Sc_(0.2)Ce_(0.01)Zr_(0.79)O_(2-x)), lanthanum strontium magnesiumgallate (LSGM, e.g., La_(0.8)Sr_(0.2)Ga_(0.8)Mg_(0.2)O_(3-x),La_(0.9)Sr_(0.1)Ga_(0.8)Mg_(0.2)O_(3-x), orLa_(0.8)Sr_(0.2)Ga_(0.83)Mg_(0.17)O_(3-x)), SDC, or GDC. Such solidelectrolytes 120 may have dopant concentrations ranging from, forexample, approximately 5 to approximately 30 mol %, or fromapproximately 10 to approximately 20 mol %. In various embodiments, thesolid electrolyte 120 may include, consist essentially of, or consist ofone or more doped cerias such as yttria-doped ceria (YDC, e.g.,Y_(0.1)Ce_(0.9)O_(1.95)), neodymium-doped ceria (NdDC, e.g.,Nd_(0.1)Ce_(0.9)O_(1.95)), praseodymium-doped ceria (PrDC, e.g.,Pr_(0.1)Ce_(0.9)O_(1.95)), and/or lanthanum-doped ceria (LaDC, e.g.,La_(0.1)Ce_(0.9)O_(1.95)). Such solid electrolytes 120 may have dopantconcentrations ranging from, for example, approximately 5 toapproximately 30 mol %, or from approximately 10 to approximately 20 mol%.

In various embodiments, the electrolyte 120 is composed of two or morelayers having different compositions. In other embodiments, theelectrolyte 120 may have a composition in which the concentration of oneor more elements, dopants, or species varies along at least a portion ofthe thickness of electrolyte 120. For example, in an SOFC 100 in whichthe functional layer 140 contains a dopant such as Co, all or a portionof the electrolyte 120 may also contain the dopant, but at aconcentration different from (e.g., less than) that at which it ispresent within the functional layer 140. For example, the electrolyte120 may have a first layer, adjacent to the functional layer 140, whichcontains a first concentration of the dopant, where the firstconcentration is less than the concentration of the dopant within thefunctional layer 140. The electrolyte 120 may also have a second layer,adjacent to the first layer and/or to the anode 130, which contains thedopant at a second concentration less than the first concentration (andthe second concentration may even be approximately zero). Similarly, allor a portion of the thickness of the electrolyte 120 (e.g., a portion ofthe electrolyte 120 adjacent to the functional layer 140, or even theentire electrolyte 120) may have a varying (e.g., graded) composition ofthe dopant that decreases as a function of distance away from thefunctional layer 140. Such compositional profiles may resistinterdiffusion within the various layers of the SOFC 100, therebymaintaining high SOFC performance.

Like the cathode 110, the anode 130 is preferably a porous conductor ofboth ions and electrons in order to promote the electrochemicalreaction. In various embodiments of the invention, the anode 130includes, consists essentially of, or consists of, for example, acomposite containing a nickel oxide and one or more oxide materials(e.g., one or more materials present within electrolyte 120, althoughsuch materials need not be preset in electrolyte 120). Other oxidematerials present within anodes 130 in accordance with embodiments ofthe invention may include yttria-stabilized zirconia (YSZ) ranging inyttria content from 3 to 8 mol %, a doped ceria-based material, and/or aspecific doped ceria material found in the solid electrolyte 120 (e.g.,GDC). For example, the anode 130 may include, consist essentially of, orconsist of a cermet of nickel oxide and yttria-stabilized zirconia. Insuch a cermet the nickel oxide to yttria-stabilized zirconia ratio mayrange from, for example, approximately 1:2 to 3:1 by weight. In anotherexample, the anode 130 may include, consist essentially of, or consistof a cermet of nickel oxide and GDC. In other embodiments, the anode 130may include, consist essentially of, or consist of a ceramic oxidematerial containing strontium, iron, cobalt, and molybdenum (i.e.,SCFM), for example SrFe_(0.1)Co_(0.45)Mo_(0.45)O₃,SrFe_(0.2)Co_(0.4)Mo_(0.4)O₃, SrFe_(0.34)Co_(0.33)Mo_(0.33)O₃, orSrFe_(0.5)Co_(0.25)Mo_(0.25)O₃.

In various embodiments, the SOFC 100 may include an anode functionallayer (not depicted in FIG. 1) disposed between the anode 130 and theelectrolyte 120. The anode functional layer may be, for example, amixture of the anode material and a ceria-based electrolyte. Forexample, the anode functional layer may include, consist essentially of,or consist of a cermet of nickel oxide and GDC. In such a cermet thenickel oxide to GDC ratio may range from, for example, approximately 1:2to 3:1 by weight. In another example, the anode functional layer mayinclude, consist essentially of, or consist of a ceramic oxide materialcontaining strontium, iron, cobalt, and molybdenum (i.e., SCFM), forexample SrFe_(0.1)Co_(0.45)Mo_(0.45)O₃, SrFe_(0.2)Co_(0.4)Mo_(0.4)O₃,SrFe_(0.34)Co_(0.33)Mo_(0.33)O₃, or SrFe_(0.5)Co_(0.25)Mo_(0.25)O₃, andGDC. In such a cermet the SFCM to GDC ratio may range from, for example,approximately 1:2 to 3:1 by weight. In various embodiments, thethickness of the anode functional layer may range from approximately5 μmto approximately 50 μm, or from approximately 10 μm to approximately 25μm.

In accordance with various embodiments of the present invention, thefunctional layer 140 is disposed at the interface between the cathode110 and the solid electrolyte 120 and reduces the interfacial resistance(e.g., to ionic conduction) therebetween. In various embodiments, thefunctional layer 140 includes, consists essentially of, or consists of aceramic material doped with one or more metals (e.g., cobalt). Thefunctional layer 140 may contain cobalt and/or one or more other metalsin the range of, for example, 0.1 mol % to 10 mol % of the ceramicmaterial. In various embodiments, the cobalt and/or other metalconcentration ranges from 0.5 mol % to 5 mol % of the ceramic material.In various embodiments, the ceramic material of the functional layer 140is one or more materials of the cathode 110 and/or of the solidelectrolyte 120. For example, the functional layer 140 may include,consist essentially of, or consist of cobalt-doped GDC and/orcobalt-doped SDC. In addition, the doped ceria composition in thefunctional layer 140 may have a different dopant than the electrolyte.For example, the functional layer 140 may include, consist essentiallyof, or consist of cobalt-doped SDC while the electrolyte may include,consist essentially of, or consist of GDC.

Various techniques may be utilized for the fabrication of the SOFC 100and its constituent layers in accordance with embodiments of the presentinvention. For example, the anode 130 and/or the solid electrolyte 120may be fabricated by tape casting of the individual layers. In variousembodiments, the anode 130 and solid electrolyte 120 are laminatedtogether by, for example, hot isostatic pressing, before application ofthe functional layer 140. The functional layer 140 and/or the cathode110 may be applied as a suspension or paste (e.g., mixed with one ormore liquid agents) via, e.g., drop coating, spin coating, spraycoating, blade coating, or screen printing. In addition, the functionallayer 140 may be deposited by physical deposition (PVD) techniquesincluding sputtering, evaporation, and pulsed laser deposition (PLD).Such techniques may be advantageous for the formation of thin layershaving thicknesses less than 5 μm, or even less than 1 μm. In variousembodiments, the functional layer 140 and the cathode 110 layer areco-fired, i.e., subjected together to a single annealing step. In aco-fired process the functional layer 140 is first deposited on the SOFC100, followed by deposition of the cathode 110 on the functional layer140. Then, the SOFC 100 with both layers is co-fired, annealing bothlayers with the same thermal exposure. For example, the SOFC 100 may beannealed at a temperature ranging from approximately 800° C. toapproximately 1100° C. and/or for a time period ranging fromapproximately 0.5 hour to approximately 5 hours.

Alternatively, the functional layer 140 may be applied and then annealedfor sintering before application of the cathode 110. In such as processthe functional layer 140 and the cathode 110 may be fired in separateannealing steps to allow optimization of the materials parameters of theindividual layers, or for other production considerations such asminimization of contamination. After application of the functional layer140, the partial SOFC 100 (i.e., without the cathode 110) may beannealed. For example, the partial SOFC 100 may be annealed at atemperature ranging from approximately 800° C. to approximately 1100° C.and/or for a time period ranging from approximately 0.5 hour toapproximately 5 hours. Then, after application of the cathode 110, theentire SOFC 100 may be annealed. For example, the SOFC 100 may beannealed at a temperature ranging from approximately 800° C. toapproximately 1100° C. and/or for a time period ranging fromapproximately 0.5 hour to approximately 5 hours.

Various functional layers 140 in accordance with embodiments of theinvention contain species such as Co that may act as sintering agents,thereby enabling the use of sintering anneals at lower temperaturesand/or for shorter times. For example, Co-containing functional layers140 may be sintered at temperatures of approximately 950° C. or below,or even 900° C. or below, while still achieving sufficient layer densityto enable high performance of SOFC 100.

EXAMPLES

In order to evaluate the effect of the functional layer 140 on cathodeperformance, a functional layer 140 composed of cobalt-doped GDC wasprepared by solid-state synthesis. Specifically, GDC(Ce_(0.9)Gd_(0.1)O_(1.95)) and cobalt oxide (Co₃O₄) (here 2 mol %;embodiments of the invention may utilize, e.g., approximately 0.1 mol %to approximately 10 mol %) were mixed and ball milled for a period of 24hours. The resulting mixture was dried, heat treated at 700° C. for 2hours, and then ball milled again for 24 hours to produce thecobalt-doped GDC.

In order to study the effect of this functional layer 140 on cathodeperformance, symmetrical test structures were fabricated, as shown inFIGS. 2A and 2B. As shown, test structure 200 featured a cathode 110 oneither side of a solid electrolyte 120. The test structure 210 wasidentical to test structure 200, but with the addition of thecobalt-doped GDC functional layer 140 between each of the cathodes 110and the solid electrolyte 120. In this series of tests, GDC was utilizedas the solid electrode 120, while a mixture of SSC(Sm_(0.5)Sr_(0.5)CoO_(3-δ)) and GDC (Ce_(0.9)Gd_(0.1)O_(1.95)) (SSC-GDC)in a ratio of 7:3 by mass or LSCF(La_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O_(3-δ)) and GDC(Ce_(0.9)Gd_(0.1)O_(1.95)) in a ratio of 1:1 by mass (LSCF-GDC) wasutilized for the cathodes 110. Once the functional layers 140 and thecathodes 110 were applied to the electrolytes 120 in the test structures200, 210, the structures were co-fired at temperatures ranging from 800°C. to 1100° C. for a period of 2 hours.

FIG. 3 is a graph of the area specific resistance (ASR) of the cathode110 for various test structures 200, 210 as a function of operatingtemperature. Curve 300 represents test structure 200 (i.e., without thefunctional layer 140) utilizing LSCF-GDC as the cathode 110 afterco-firing at a temperature of 1100° C. Curve 310 represents teststructure 200 (i.e., without the functional layer 140) utilizing SSC-GDCas the cathode 110 after co-firing at a temperature of 950° C. Curve 320represents test structure 210 (i.e., with the functional layer 140)utilizing SSC-GDC as the cathode 110 after co-firing at a temperature of900° C. Curve 330 represents test structure 210 (i.e., with thefunctional layer 140) utilizing SSC-GDC as the cathode 110 afterco-firing at a temperature of 950° C. As shown, the ASR of the teststructures 210 is much improved when compared to the performance of thetest structures 200 over the entire range of operating temperaturesstudied. In addition, the presence of the functional layer 140 withinthe test structures 210 decreased cathode polarization by approximatelyone order of magnitude (10×) and decreased the activation energysignificantly.

Performance of full SOFCs incorporating the cobalt-doped GDC functionallayer 140 was also investigated. In the SOFCs, the anode was a mixtureof nickel oxide and GDC (Ni-GDC) in a ratio of 2:3 by mass, theelectrolyte was GDC 20 μm thick, and the cathode was SSC-GDC in a ratioof 7:3 by mass. These compositions and thicknesses are only exemplary;in various embodiments, the anode may include, consist essentially of,or consist of a cermet of nickel oxide and yttria-stabilized zirconia.In such a cermet the nickel oxide to yttria-stabilized zirconia ratiomay range from, for example, approximately 1:2 to 3:1 by weight. Theanode and electrolyte layers were fabricated via tape casting and thenlaminated together using a hot press. The resulting anode/electrolyteassemblies were annealed at 1450° C. for a time period of 4 hours. Thefunctional layer was deposited on the sintered half-cell by drop coatingto a thickness ranging from 1 μm to 10 μm. The SSC-GDC cathode materialwas applied directly to the functional layer, and the resultingassemblies were co-fired at 950° C. for a time period of 2 hours. Inaddition, control SOFCs lacking the cobalt-doped GDC functional layerwere similarly fabricated for comparative purposes. The various SOFCswere evaluated at a range of temperatures utilizing humidified hydrogengas as fuel.

FIG. 4 is a graph of SOFC performance at temperatures ranging from 500°C. to 650° C. for a SOFC incorporating a cobalt-doped GDC functionallayer having a thickness of 6 μm. Curves 400, 410, 420, 430 indicate thecell voltage (in volts) as a function of current density (in amps/cm²)for operating temperatures of 500° C., 550° C., 600° C., and 650° C.respectively. Curves 440, 450, 460, 470 indicate the power density (inwatts/cm²) as a function of current density (in amps/cm²) for operatingtemperatures of 500° C., 550° C., 600° C., and 650° C. respectively.

FIG. 5 is a graph of SOFC performance at temperatures ranging from 500°C. to 650° C. for a SOFC incorporating a cobalt-doped GDC functionallayer having a thickness of 1 μm. Curves 500, 510, 520, 530 indicate thecell voltage (in volts) as a function of current density (in amps/cm²)for operating temperatures of 500° C., 550° C., 600° C., and 650° C.respectively. Curves 540, 550, 560, 570 indicate the power density (inwatts/cm²) as a function of current density (in amps/cm²) for operatingtemperatures of 500° C., 550° C., 600° C., and 650° C. respectively.

FIG. 6 is a graph of SOFC performance as a function of cobalt-doped GDCfunctional layer thickness at an operating temperature of 650° C. Curves600, 610, 620 indicate the cell voltage (in volts) as a function ofcurrent density (in amps/cm²) for SOFCs having no functional layer(i.e., a functional layer thickness of zero) and having functionallayers of thicknesses 1 μm and 6 μm, respectively. Curves 630, 640, 650indicate the power density (in watts/cm²) as a function of currentdensity (in amps/cm²) for SOFCs having no functional layer and havingfunctional layers of thicknesses 1 μm and 6 μm, respectively.

FIG. 7 is a graph of SOFC performance as a function of cobalt-doped GDCfunctional layer thickness at an operating temperature of 500° C. Curves700, 710, 720 indicate the cell voltage (in volts) as a function ofcurrent density (in amps/cm²) for SOFCs having no functional layer andhaving functional layers of thicknesses 1 μm and 6 μm, respectively.Curves 730, 740, 750 indicate the power density (in watts/cm²) as afunction of current density (in amps/cm²) for SOFCs having no functionallayer and having functional layers of thicknesses 1 μm and 6 μm,respectively.

As shown in FIGS. 4-7, the use of a functional layer 140 in accordancewith embodiments of the present invention can improve performance of anSOFC dramatically. For example, as shown in FIGS. 4 and 6, the SOFChaving the 6 μm thick functional layer exhibits an output power densityof 1.8 W/cm² at an operating temperature of 650° C., dramatically higherthan the performance of the SOFC without the functional layer. Thethicker functional layer, which improves open circuit voltage, performsbetter within this regime in which performance is limited by opencircuit voltage. Moreover, as shown in FIGS. 5 and 7, at lower operatingtemperatures (500° C.-600° C.), the SOFC having the 1 μm thickfunctional layer exhibits superior performance primarily due to thereduced ohmic resistance within the layer, even though the open circuitvoltage is not greatly improved. Thus, functional layers in accordancewith embodiments of the invention may have their thicknesses selectedbased on, at least in part, the intended operating temperature of theSOFC. Since the functional layer may improve performance viaimprovements in open circuit voltage and/or ohmic resistance, functionallayers of various thicknesses may be utilized to improve SOFCperformance over a range of different operating conditions.

FIG. 8 is a graph of oxygen surface exchange coefficients of anexemplary functional layer material (Co-GDC) and an exemplaryelectrolyte material (GDC). Here, the Co-GDC layer contains 2 mol % Co,and, as shown, the Co-GDC layer exhibits a higher oxygen surfaceexchange coefficient over the entire temperature range of approximately400° C. to approximately 750° C. In various embodiments, this higheroxygen surface exchange coefficient is due at least in part toagglomeration or enrichment of the Co at grain boundaries within theGDC. The higher oxygen surface exchange coefficient of the functionallayer enhances mass transport at the interface between cathode andelectrolyte, reducing interfacial resistance to oxygen introduction inthe electrolyte, increasing cell performance, and decreasing ASR.Moreover, the use of such functional layers in accordance withembodiments of the invention enables enhanced SOFC performance at lowertemperatures, for example between approximately 400° C. andapproximately 750° C.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A method of fabricating a solid oxide fuel cell,the method comprising: providing an anode layer; disposing a solidelectrolyte layer over the anode layer; depositing a functional layerover the solid electrolyte layer; and disposing a cathode layer over thefunctional layer, thereby forming the solid oxide fuel cell.
 2. Themethod of claim 1, wherein the solid electrolyte layer is disposed overthe anode layer before the functional layer is deposited.
 3. The methodof claim 1, wherein the anode layer is provided by tape casting.
 4. Themethod of claim 1, wherein disposing the solid electrolyte layer overthe anode layer comprises laminating the solid electrolyte layer to theanode layer.
 5. The method of claim 1, further comprising providing thesolid electrolyte layer before disposing the solid electrolyte layerover the anode layer.
 6. The method of claim 5, wherein providing thesolid electrolyte layer comprises tape casting.
 7. The method of claim1, further comprising annealing the solid oxide fuel cell after thecathode layer is disposed over the functional layer.
 8. The method ofclaim 7, wherein at least a portion of the solid oxide fuel cell isannealed at a temperature ranging from approximately 800° C. toapproximately 1100° C. for a time period ranging from approximately 0.5hour to approximately 5 hours after the solid electrolyte layer isdisposed over the anode layer.
 9. The method of claim 1, wherein thefunctional layer comprises at least one of cobalt-doped gadolinium-dopedceria or cobalt-doped samarium-doped ceria.
 10. The method of claim 1,wherein a thickness of the functional layer ranges from approximately 1μm to approximately 10 μm.
 11. The method of claim 1, wherein thefunctional layer contains cobalt at a composition ranging fromapproximately 0.5 mol % to 5 mol %.
 12. The method of claim 1, wherein(i) the functional layer contains cobalt and (ii) at least a portion ofthe solid electrolyte layer contains cobalt.
 13. The method of claim 12,wherein a concentration of cobalt in the at least a portion of the solidelectrolyte layer is less than a concentration of cobalt in thefunctional layer.
 14. The method of claim 1, further comprisingselecting a thickness of the functional layer based at least in part onan intended temperature of operation of the solid oxide fuel cell. 15.The method of claim 1, further comprising annealing the functional layerand the cathode layer together after the cathode layer is disposed overthe functional layer.
 16. The method of claim 15, wherein the annealingis performed at a temperature ranging from approximately 800° C. toapproximately 1100° C. for a time period ranging from approximately 0.5hour to approximately 5 hours.